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When you do some good to some one, don't boast of it. Think that yu have thrown in to the river. It means do not take any pride or feel important for doing any good to any one. This is the preaching to prevent you from falling in to the evil of vain pride. Pride is vanity which itself is an evil.
The same thing Geeta has taught us. KARMANI (in discharging duty) EV (only) ADHIKARAH TE (YOUR RIGHT ) = YOU have a right to discharge your duty only; not in expecting the desired results, and whatever the results, not to be attached with them.